This genetic toxicology testing contract has supported the overall mission of NTP to provide detailed toxicological profiles, including potential for carcinogenicity, of compounds of interest for 31 years. This Genetic Toxicity Testing contract provides information on exposure-related genetic damage to NTP using standard tests as well as innovative protocols when necessary. The capabilities of the contract are continuously updated to remain current and compatible with international guidelines for conducting these kinds of tests. Testing systems employed include both in vitro (animal cell-based and bacterial) and in vivo (rats and mice) assays. In vivo studies using human blood samples can also be conducted to translate findings from animal studies to human exposure scenarios. Three main tests are conducted routinely: in vitro bacterial mutagenicity assays, in vivo rodent peripheral blood micronucleus (MN) assays, and in vivo rodent DNA damage (Comet) assays in multiple tissues including, for example, liver, brain, stomach, kidney, and lung. Using peripheral blood as the sample source, MN and Comet studies can also be conducted in humans. In addition, in vitro MN assays have become increasingly used as a substitute for stand-alone in vivo rodent MN assays. An animal mutation endpoint that holds promise for application in human clinical and biomonitoring studies in the future, is now routinely assessed in the testing laboratory: the pig-a mutation assay (phosphatidylinositol glycan anchor biosynthesis, class A gene). Mutations in this gene are easily detected in red blood cell samples from laboratory rodents, and updated methods to streamline the assay have been developed, so that integration into existing toxicity tests can easily be accomplished. We have started to multiplex this assay with the in vivo MN and Comet assays, increasing the genetic damage information we obtain from test animals to provide an even more comprehensive profile of the genetic toxicity potential of a chemical. In addition to our cell-based and animal model studies, human blood samples provided by collaborators such as NIOSH and the NIEHS Clinical Research Unit may also be examined for specific genetic endpoints following an environmental exposure that is suspected of inducing genetic damage, or has been shown to do so in NTP animal studies. The number of studies of each type described above varies annually, depending on the needs of the NTP. Historical control data bases have been established for MN and pig-a data from a new strain of mouse ? the CD-1 mouse ? that is used in NTP reproductive (transgenerational) studies. In addition, a pig-a historical control database has been established for the Sprague Dawley rat model in these same types of studies. We now are able to place our rat pig-a and our CD-1 mouse MN and pig-a data into context, which is necessary for the interpretation of the observed responses.